A former communication terminal apparatus having external attachment telephone, detects a hookup of the external attachment telephone through a change of line voltage. That is, the former communication terminal apparatus samples the line voltage that is supplied to line, and compares the sampled line voltage (as a following line voltage) with a line voltage (as a previous line voltage) that was sampled a certain time earlier than the following line voltage. Then, the former communication terminal apparatus detects that the external attachment telephone is hooked up when the number of times counted in the case that a voltage difference between the following line voltage and the previous line voltage sampled a certain time earlier than the following line voltage becomes a predetermined threshold value or over, continuously reaches a predetermined number of times (for example, refer to patent document 1).    Patent document 1: Japan patent publication Hei8-289039.
The communication terminal apparatus is connected with a line exchange machine. As the line exchange machine, for example, a subscriber line exchange machine furnished by an electric communication business company; and a private line exchange machine (PBX) furnished in branch by user are adopted. In these line exchange machines, line voltage that is supplied to line between the line exchange machine and the communication terminal apparatus is different. Here, the subscriber line exchange machine uses high line voltage; the private line exchange machine uses low line voltage.
The former communication terminal apparatus stated above, receives a ring signal from the line exchange machine according to a predetermined time interval. In the communication terminal apparatus, from the time that the ring signal is received to the time that a next ring signal is received, through the change of line voltage, such case occurs that the number of times counted when a voltage difference between the following line voltage and the previous line voltage sampled a certain time earlier than the following line voltage becomes a predetermined threshold value or over, continuously reaches a predetermined number of times. Thus, in the former communication terminal apparatus, though the external attachment telephone is not hooked up in fact, it is misunderstood that it has been hooked up and is not shifted to a receiving mode. Especially, when the line exchange machine whose line voltage is low is compared with the line exchange machine whose line voltage is high, in the case to receive a change of the same line voltage, because the ratio of the voltage that has changed becomes higher with respect to the threshold value, it is mostly misunderstood that the external attachment telephone was hooked up.